Testing for carbonyls

Question 1

,Main test to see if a C=O bond is present

Answer 1

2,4-DNPH test

Question 2

What does 2,4-DNPH have a positive test for?

Answer 2

Both aldehydes and ketones

Question 3

Conditions for 2,4-DNPH test

Answer 3

Concentrated sulfuric acid
dissolved in methanol
Warmed

Question 4

Positive result for 2,4-DNPH test

Answer 4

A bright orange precipitate is formed which are crystals

Question 5

What can be done after the 2,4-DNPH test?

Answer 5

Purify crystals by recrystallisation
Complete melting point test to find MP
Compare to data booklet values of MP to identify the carbonyl we had

Question 6

Does the 2,4-DNPH test differentiate aldehydes from ketones?

Answer 6

NO after a positive result for 2,4-DNPH test we can complete after tests to differentiate

Question 7

3 tests that give a positive result for aldehydes but NOT ketones

Answer 7

Tollens’ reagent
Fehling’s/ Benedict’s solution
Acidified dichromate (VI) ions

Question 8

Why can the 3 tests distinguish an aldehyde from ketone by giving a positive result for the aldehyde?

Answer 8

Because aldehydes can be oxidised again to carboxylic acids whereas ketones can only be oxidised by breaking C-C bonds
Positive result = shows substance was oxidised (aldehyde)

Question 9

How to represent oxidising agent?

Answer 9

[O]

Question 10

What is Tollens’ reagent?

Answer 10

Colourless silver nitrate solution dissolved in aqueous ammonia
Complex ion [Ag(NH3)2]+

Question 11

Another name for Tollen’s reagent test

Answer 11

Silver mirror test

Question 12

Conditions for Tollens’ reagent test

Answer 12

Heat the silver nitrate solution in aqueous ammonia with an aldehyde

Question 13

Oxidising agent in Tollens’ reagent

Answer 13

Ag+ ions are reduced to Ag
And oxidises the aldehyde

Question 14

What are aldehydes oxidised to when testing for them?

Answer 14

Carboxylic acids

Question 15

What observation indicates a positive result for Tollens’ reagent?

Answer 15

A silver mirror formed which coats test tube as Ag+ ions were reduced to Ag atoms

Question 16

Word equation for Tollens’ reagent

Answer 16

[Ag(NH3)2]+ + Aldehyde + water ——> silver + carboxylic acid + H+ ions

Question 17

What is Fehling’s solution?

Answer 17

Cu 2+ ions dissolved in NaOH

Question 18

What is Benedict’s solution?

Answer 18

Cu 2+ ions dissolved in Na2CO3 solution

Question 19

Oxidising agent in Benedict’s/Fehling’s solution

Answer 19

Cu 2+ ions that are reduced to Cu2O (Cu 1+ ions)

Question 20

Observation for positive result of Benedict’s/Fehling’s test

Answer 20

Colour change blue solution to form red precipitate of copper oxide

Question 21

Word equation for Fehling’s/Benedict’s?

Answer 21

Aldehyde + [O] ——-> Carboxylic acid + copper oxide

Question 22

What is Acidified dichromate ions?

Answer 22

H+/Cr2O7 2- ions

Question 23

Oxidising agent for acidified dichromate test?

Answer 23

Cr2O7 2-
The Cr 6+ ions are reduced to Cr 3+ ions

Question 24

Observation for positive result acidified dichromate ions test?

Answer 24

Colour change orange to green

Question 25

Iodoform test reagent

Answer 25

Iodine in presence of cold aqueous hydroxide alkali

Question 26

What does iodoform give a positive result for?

Answer 26

Methyl carbonyl groups, the carbonyl group is bonded to a methyl group which is CH3
O=C-CH3

Question 27

What molecules give a positive result for the iodoform test?

Answer 27

Ethanal
Any ketone which C=O is bonded to the 2nd carbon so there is a methyl group to be the end of the chain

Question 28

Positive result for iodoform test

Answer 28

Yellow precipitate of CH3I
And antiseptic smell

Question 29

Why do only methyl carbonyl groups give a positive iodoform test result?

Answer 29

Because a yellow precipitate of CH3I was formed